Today, image forming apparatuses that form images on sheets such as printers, facsimile devices, and copying machines are equipped with units that are removably mounted therein. These units can be detached by being slid relative to the main bodies of the image forming apparatuses (hereinafter referred to as the main bodies of the apparatuses). Such units include, for example, sheet feeding units that contain sheets, sheet feeding devices that feed sheets from sheet feeding cassettes, and image forming portions (transfer unit, fixing unit, and the like) that form images on sheets.
After such a unit is pulled out from the main body of the apparatus, the user manually pushes the unit back into the main body of the apparatus in order to mount the unit in the main body of the apparatus. However, sometimes the unit is not sufficiently pushed to a position that is reached when the unit is completely mounted in the main body of the apparatus. In such a case in which the unit is not fully mounted in the main body of the apparatus, when the unit is, for example, a sheet feeding cassette, sheets are not correctly positioned. Thus, situations such as an image being incorrectly formed on the sheet or defective feeding of the sheet (the sheet being fed in a tilted position or sheet jamming) can occur. When the unit is a transfer unit or the like that transfers an image onto the sheet, quality of an image is degraded due to a transfer of the image onto an incorrect position on a transported sheet.
In view of the above-described situation, technologies using mechanisms that assist an operation of pulling the unit have been proposed in order to improve operational properties in mounting and removing the unit by the user. For example, PTL 1 proposes a technology using a unit moving device in a case in which the unit is a sheet feeding cassette. With this technology, the sheet feeding cassette is automatically forced to be pulled to a normal mounting position of the sheet feeding cassette in a main body of an apparatus by a driving unit such as a motor from a position located in front of the normal mounting position.
PTL 2 proposes another unit moving device. According to the document, a protrusion is provided in the sheet feeding cassette, and a rotating body urged by a spring is provided on the side of a main body of an apparatus. When the protrusion goes beyond the rotating body, an elastic force of the spring causes the rotating body to urge the protrusion. In this unit moving device, by urging the sheet feeding device using the protrusion as described above, the sheet feeding cassette is forced to be pulled to a mounting position from a position located in front of the mounting position.
However, there is a problem with the above-described related-art unit moving device. The device uses the driving unit, for example, a motor or the like, in order to force the sheet feeding cassette to be pulled to the normal mounting position in the main body of the apparatus. This makes the structure be complex, thereby increasing the size and cost of the apparatus.
The other related-art unit moving device pulls the sheet feeding cassette using the rotating body urged by the spring. In this case, the size and cost of the apparatus, which increase in the case of the apparatus using the motor, do not increase. However, today, in order to decrease the number of times of replenishment of the sheets in the sheet feeding cassette so as to improve work efficiency of the user, the amount (number) of sheets loaded in the sheet feeding cassette tends to increase. For this reason, the total weight of the sheet feeding cassette loaded with the maximum amount of the sheets increases. Thus, in order to reliably pull the sheet feeding cassette to the normal mounting position in the main body of the apparatus, use of a spring having a large elastic force is highly desirable.
When a spring having a large elastic force is used, in order to move the sheet feeding cassette from the main body of the apparatus, the operation needs be performed against a large spring force. Thus, a larger operational force is required, and accordingly, operability is degraded.
Although the operability of the sheet feeding cassette is described above, similar problems occur in a transfer portion that transfers toner images onto a sheet, or in a transfer unit or a fixing unit of which a fixing device fixing toner images to a sheet can be pulled from the main body of the apparatus. Recently, the number of image forming apparatuses able to form color images has been increasing. Accordingly, in an image forming portion of an image forming apparatus, a plurality of photosensitive drums and a fixing device having a large heat capacity are used. This also increases the size and weight of the image forming portion. For this reason, degradation of work efficiency also occurs in structures that pull these units as is the case with a structure that pulls the sheet feeding cassette.